The present invention relates to an ATM (asynchronous transfer mode) switching system that is a backbone member in communication across a multimedia network, and particularly, to improvement in switching performance between an act-system and standby-system communication channels placed as a redundant system in the ATM switching system.
In general, to make sure that communication is securely performed, an ATM has a duplex (redundant) system, which consists of, for example, an act and standby communication systems (channels) including switches for switching the channels. Cells are sent to both the act and standby systems, and cells passing through the act system are handled as effective ones in a normal state. However, when something accidental occasions occur in the act system, the switches are switched to the standby system.
Although cells are sent synchronously with clocks in both the act and standby systems, the cells may slightly shifted from each other in timing. This shift may result in a partial loss of the cells (i.e., cell losses) during the switching operation of the switches. If the receiver is under reception of image data, for example, the cell losses result in influences such as skips or disturbances of the images.
Thus, it is required to avoid such cell losses (referred to as xe2x80x9cnon momentary interruption performancexe2x80x9d) during the switching operation of the switches.
In many conventional ATM switching systems, the non momentary interruption performance for cell communication during the switching is taken for granted, in addition to a precondition that a normal cell communication is secured even after the switching.
However, in such conventional ATM switching systems, since an ultra high speed transfer of fixed length cells is carried out, subtle disturbances in clocks supplied to each unit composing the switch systems cause various drawbacks such as cell losses or stops in a stream of cells (cell stops). Large numbers of cell losses or cell stops responding to the switching has been on the rise, which may have been resulted in interruptions of ATM service.
The present invention is realized to overcome the above drawbacks. An object of the present invention is to provide an ATM switching system capable of performing a smooth and stable switching between an act system and standby system communication channels configured therein, without causing such problems as cell losses.
In order to accomplish the above object, an ATM switching system of the present invention is constructed as follows. The ATM switching system comprises a redundant communication system consisting of an act-system and standby-system communication channels, a cell being sent through both the communication channels, and a switch element for performing system switching from the act-system communication channel to the standby-system communication channel in response to a request-to-switch, said switch element connecting with a line concentrator. Normally, cells are sent through the act-system communication channel to a destination to be sent. In this state, if a request-to-switch is generated due to the fact that abnormalities or the like occur in the act-system communication channel, the path is switched from the act-system communication channel to that of the standby-system by the switch element. The switching permits cells which pass the standby-system communication channel to be sent to the destination.
In the ATM switching system, an aspect of the present invention is that it has an element for reading, when receiving the request-to switch, a number of the cell which passes the line concentrator for each of the act-system and standby-system communication channels, and an element for controlling a switching operation of the switch element based on the read numbers of the passing cell for both the communication channels.
Preferably, the controlling element comprises an element for calculating a difference between the read number of the passing cell in the act-system communication channel and the read number of the passing cell in the standby-system communication channel, an element for allowing the switch element to perform the switching operation when the difference is less than a specified value, and an element for prohibiting the switch element from the switching operation when the difference is not less than the specified value.
It is preferred that the switching controlling element comprises an element for notifying an administrator of the fact that the system switching has not been accomplished when the switching has not been allowed.
Moreover, the reading element is possible to be constructed such that the number of passing cells for each of the act-system and standby-system communication channels is read specified times, preferably, specified times at intervals. Compared to one time of the reading, objects to be determined increase in number, making it more reliable the switching determination by the controlling element.
Moreover, for each communication channels consisting of a plurality of channels, it is preferred that the switching determination be applied to all the channels and those determination results be reflected in its final switching control.
To be specific, since an ATM switching system has a plurality of information highways for each of the act-system and standby-system communication channels, preferred is that the passing cell number is measured (read) for each information highway to determine the final switching, thus increasing reliability in the switching.
Instead of reading the cell number formation highway, as an communication channel, the reading element may be formed into a construction having an element reading, preferably, specified times at intervals, the number of the passing cell every logic path (VPI/VCI) of the line concentrator for each of the act-system and standby-system communication channels.
In addition, another aspect of the above ATM switching system is that it comprises an element for sending, when receiving the request-to-switch, a specified cell from the first line concentrator placed in the standby-system communication channel to the standby-system communication channel of the second line concentrator, at least one time , preferably, specified times at intervals, and an element for controlling a switching operation of the switch element by not only allowing the switching operation when a number of the received specified cell in the standby-system communication channel of the second line concentrator and a number of the sent specified cell equal to each other but also prohibiting the switching operation when both the numbers differ from each other
In this construction, it is also preferred that the switching controlling element comprises an element for notifying an administrator of the fact that the system switching has not been accomplished for avoiding cell losses when the switching has not been allowed.
By way of example, the specified cell sending element includes an element storing a checking data into the specified cell when sending the specified cell, and said switching controlling element include an element not only allowing the switching operation when the checking data in the specified cell received at the standby-system communication channel of the second line concentrator agree with an original data concerning the checking data but also prohibiting the switching operation when the checking data differ from the original data. This gives a steady switching manner to the switch element. A check sum value stored in a cell payload part of the specified cell is one example of the checking data.
In the invention, for switching from the act system to the standby system, the numbers of cells passing the individual act and standby systems are read for searching a condition in which both the cell numbers are as close as possible to each other. When such condition is realized, the switching is performed, avoiding occurrence of cell losses as much as possible.
Further, by way of example, the switching from the act system to the standby system is done only when a specified cell is sent from the first line concentrator in the standby system to the standby system of the second line concentrator, and the number of received specified cells in the standby system of the second line concentrator agrees with its sending line concentrator and a check sum value of a cell payload part of the specified cell agrees with its original data, preventing cell losses as well.